Olefin copolymers, including semi crystalline ethylene-propylene copolymers, have been widely used as a class of viscosity index improvers for lubricating oils. In these uses, the performance of these polymers is typically measured by the ratio of the thickening efficiency (TE) to the shear stability index (SSI). Olefin copolymers with an SSI of from about 35% to about 50% are typically known as high SSI viscosity index improvers, and are known to deliver exceptional fuel economy and thickening power, compared to lower SSI materials.
These olefin copolymers, however, are generally deficient in low temperature properties and exhibit inadequate “flowability” and “pumpability” as measured by pour point, and low temperature viscosity in a Mini Rotary Viscometer (MRV) test. Furthermore, high ethylene olefin copolymers are generally unsuitable for waxy base stock oils, especially Group I AC 150 oils.
Semi crystalline ethylene-propylene copolymers having ethylene contents of 53% to 57 wt % that are made in a single catalyst, single reactor environment, however, typically gel at low temperatures or have unacceptably high MRV or Cold Cranking Simulator (CCS) viscosity values. If the ethylene content is lower (such as 45% to 50 wt %) the single catalyst, single reactor polymers are very difficult to handle because of their amorphous nature and their tendency to agglomerate and stick to the finishing equipment.
To avoid these drawbacks, more complicated and cumbersome processes, including shear processes, bimodal blends, and blocky copolymers, have been used. Some illustrative processes are described in U.S. Pat. Nos. 4,540,753; 4,804,794; 5,391,617; 5,451,630; 5,451,636; 5,837,773; 6,753,381; U.S. Patent Publication No. 20030176579; WO2006102146; EP1148115; EP1178102; and EP1262498.
There is a need, therefore, for high SSI viscosity index improvers that exhibit excellent low temperature properties without detrimentally affecting fuel economy and thickening power, and that are stable in the finishing process in a single reactor environment (with single catalyst) without having to blend and shear the polymer or use other multiple step processes.